1. Field of the Invention
The present invention relates to an injection molding machine and, more particularly, to an improvement of a method for controlling drive of a screw in a metering or dispensing process.
2. Description of the Related Art
Motor-driven molding machines have increasingly been used in recent years in which a servo motor is used as a substitute for a hydraulic actuator. Operations of such injection device using a servo motor are summarized below.
A screw is rotated with a servo motor for screw rotation in a plasticization/metering process. The screw is located within a heating cylinder. A resin is fed from a hopper to a rear portion of the screw in the heating cylinder. Rotation of the screw melts and advances the resin and thus a certain metered amount of resin is fed to a nose portion of the heating cylinder. During this time, the screw is driven backward due to a back pressure of the molten resin trapped in the nose portion of the heating cylinder.
An injection shaft is directly connected to the rear end portion of the screw. The injection shaft is rotatably supported by a pressure plate through a bearing. The injection shaft is driven in the axial direction with a servo motor for injection that is supported on the pressure plate. The pressure plate moves forward and backward along guide bars in response to the operation of the servo motor for injection through a ball screw. The above-mentioned back pressure of the molten resin is detected by using a load cell and is controlled with a control loop, as will be described more in detail below.
Then, in a filling process, the pressure plate is advanced by means of driving the servo motor for injection. A nose portion of the screw serves as a piston to fill a mold with the molten resin.
The molten resin fills the space within the cavity at the end of the filling process. At this point, the control mode for the advancing motion of the screw is switched from a velocity control mode to a pressure control mode. This switching is referred to as a "V (Velocity)-P (Pressure) switching".
Following the V-P switching, the resin within the cavity of the mold is allowed to cool under a predetermined pressure. This process is referred to as a dwelling process. In this dwelling process, the pressure of the resin is controlled in a feedback control loop as in the above-mentioned back pressure control.
Then, the injection device returns to the plasticization/metering process after the completion of the dwelling process. On the other hand, in a clamping device, an eject operation is carried out for ejecting a solid product out of the mold in parallel with the plasticization/metering process. The eject operation involves in opening the mold to remove the solid product from the mold by means of an ejector mechanism and then closing the mold for the resin filling.
As will be described more in detail below, a rotation speed of the screw in the metering process is determined such that the screw rotates at the highest speed at the beginning of the metering, which is decreased stepwise during the course of the metering. In particular, it is necessary to use a linear or quadric function to slow down an initial (start-up) rotation of the screw at the beginning of the metering process until the rotation speed reaches a predetermined value N. To this end, the start-up operation should be slowed down until the speed reaches the predetermined value N. This prolongs the time duration before reaching the predetermined value N and, in turn, prolongs one molding cycle.
In addition, the rotation speed of the screw during the metering process is required to be determined stepwise depending on a stroke, that is, a position of the screw while considering a resident time of the resin. Having such several values for the rotation speed is troublesome and bothering because it should be done when it becomes necessary to change a diameter of the screw.